It is known that a saturable absorber is a material whose transmittance changes depending on the absorption amount of light, and serves as a Q-switched device only by being inserted into a laser resonator. The saturable absorber suppresses laser oscillation in a weakly excited state, but when a laser material is strongly excited, and a gain becomes higher than a loss in a resonator including a loss caused by the absorption by the saturable absorber, laser oscillation starts in the resonator. In this case, when the saturable absorber absorbs strong laser light, the absorption is saturated due to the depletion of lower-level ions, and the saturable absorber abruptly becomes transparent with respect to laser light. As a result of this operation, the Q-value of the resonator increases, and Q-switch oscillation occurs.
As an example of a passively Q-switched laser, there has been reported a passively Q-switched laser including a semiconductor laser, a combined optical system, a slab-type laser material, a total reflection mirror, an output mirror, and a saturable absorber (see Non Patent Literature 1).
A general solid-state pulse laser as disclosed in Non Patent Literature 1 oscillates in a great number of high-order modes. The spread of laser light oscillating in a high-order mode is spatially large, compared to laser light in a single mode, and hence only oscillation in a low-order mode may occur if an aperture is limited to be small.
Therefore, in a related-art laser device, there has been proposed a laser device which suppresses the surrounding oscillation in an unnecessary mode with a shielding plate having minute holes serving as a transverse mode selection element in a resonator (see Patent Literature 1).
Further, as an example of a related-art solid-state laser device, there has been proposed a laser device in which a center portion of an output mirror is applied with a partial reflection coat and the outer circumferential portion thereof is applied with an antireflecting coat, to thereby control a mode in a resonator (see Patent Literature 2).